In general, a numerically controlled machine tool of the prior art is capable of changing automatically a tool mounted at the front end of the main spindle with another tool kept in the tool magazine. An automatic tool change device as described above comprises two mechanisms, which include a revolving motion and m linear motion. The shortcomings inherent in these two mechanisms of the prior art machine tool are described respectively hereinafter.
One mechanism by which the tool change task is brought about is controlled by the hydraulic power in conjunction with gears,gear racks, and a hydraulic cylinder. Such mechanism is defective in that the motion characteristics of the hydraulic device cannot be controlled with ease, and that the operation of the device is unstable so as to generate an impact force at the end of the motion. Further the locating precision of the mechanism is generally poor and unreliable. Still further, the operating speed of the mechanism cannot be significantly accelerated. In addition, the maintenance of the mechanism must be done frequently.
Another mechanism, by which the tool-change action is brought about by the numerically controlled machine tool of the prior art, is controlled by a cam system. This mechanism is limited in that the linear path of the tool-switching main spindle requires a greater stroke at the time when the tool-change process is under way, thereby resulting in an excess of the pressure angle of the cam. Accordingly, a greater force must be exerted on the cam. The remedial measure of the situation described above is to enlarge the dimension of the cam; nevertheless such remedial measure results in a substantial increase in the volume of the cam and the cost of making the cam. In addition, the bulky cam lead to a space problem in the assembly of the cam.